1. Field of the Invention
This invention generally relates to a shielding structure of a through-hole formed in a plastic hollow product and in particular to a shielding structure in the case when a through-hole is formed in a duct, such as a duct of an air-intake system of an automobile engine, for detecting its interior environment and a sensor is located in the through-hole.
2. Description of the Prior Art
Heretofore, in the case of providing a through-hole in a wall of a plastic hollow product and providing a branch pipe in communication with the interior space of the hollow product via the through-hole, a technology for securing air-tightness between the hollow product and the branch pipe was known. For example, Japanese Patent Laid-open Pub. No. Sho 64-56532, assigned to the assignee of this application, discloses a sealing technology in which the sealing characteristic is improved between the plastic hollow product and the branch pipe through thermal adhesiveness. In addition, another Japanese Pat. Laid-open Pub. No. Hei 7-124999, also assigned to the assignee of this application, discloses a sealing technology in which the air-tightness between the hollow product and the branch pipe by having the branch pipe integrally molded with the plastic hollow product according to an insert molding method. Although these technologies allow to improve the sealing characteristic between a plastic hollow product and a branch pipe added thereto, they are directed to improve the air-tightness between the plastic hollow product and the branch pipe and not to sealing of a through-hole formed in a wall of a plastic hollow product and of the hollow space of a branch pipe itself.
In an air-intake duct for use in an air-intake system of an automobile engine, it is required to detect the condition of the fluid flowing through the air-intake duct. For example, in order to control the operating condition of an engine for the purpose of enhancing the fuel consumption efficiency and the purification of the exhaust gas, various kinds of sensor are used for detecting the condition of the fluid flowing through the air-intake duct. These sensors are mounted at their desired locations along the air-intake duct to detect the environment inside the air-intake duct and the detected information is then used to control the operating condition of the engine.
In this manner, components such as various kinds of sensors are required to be mounted on their desired locations along the air-intake duct, in which case, if the air-intake duct is comprised of a plastic hollow product, since its rigidity is relatively low as compared with a metal, there arises a problem of securing the air-tightness between a sensor and the air-intake duct properly. That is, in providing a component such as a sensor, it is necessary to provide a through-hole in the wall of the air-intake duct and have the component inserted into the through-hole to detect the condition inside the air-intake duct and then to provide the detected information to a controller. In this instance, since the air-intake duct is continuously subjected to vibration during the operation of the engine, in order to have the component securely held to the air-intake duct, the component is required to be fixedly attached to the air-intake duct. In addition, because of the necessity for possible replacement of the components, the component is typically fixedly attached to the air-intake duct using a nut and a bolt. In such a case, either one of a nut and a bolt has to be fixed to the air-intake duct; however, if the air-intake duct is comprised of a plastic hollow product, there is difficulty in having a nut or a bolt securely fixed in the plastic hollow product.
Moreover, when a component such as a sensor is mounted in a predetermined position as inserted into a through-hole formed in the air-intake duct, there arises difficulty in sealing the gap between the component and the through-hole. That is, if the air-intake duct is comprised of a plastic hollow product, it is required to be made to be relatively high in rigidity so that a nut or a bolt can be securely held, but in the case of a plastic hollow product which is relatively high in rigidity, there arises a problem of securely sealing the gap between the component and the through-hole because of constant application of vibration to the air-intake duct during the operation of the engine.
Therefore, there is a need to provide a sealing technology capable of securely sealing the through-hole formed in the wall of a plastic hollow product. For example, in the case of having a component, such as a sensor, mounted in the air-intake duct as inserted into the through-hole, there is a need to develop a sealing technology which allows to have the component mounted at a predetermined position securely and to have the gap between the through-hole and the component sealed securely. In particular, in the case when a plastic hollow product is used as an air-intake duct of an automobile engine, since the hollow product is constantly subjected to vibration at least during the operation of the engine, there is a need to develop a sealing technology that does not deteriorate the sealing characteristic even if such vibration is applied.